On the primary thermal decomposition pathways of hydroxycinnamic acids

Abstract

Valorization of pyrolytic lignin to fuels and chemicals is still poorly understood due to its ill-defined structure and the complexity of the decomposition chemistry. To shed some light on the dominant reaction pathways of lignin thermolysis, novel experimental and first-principles based calculations of its building blocks have been carried out. Pyrolysis chemistry of hydroxycinnamic acids is investigated in this work using a unique Py-GC × GC-FID/TOF-MS coupled with a customized GC to detect water and gases, to gain an understanding of the role of the branching ratios in lignin and its linkages with hemicellulose. Mean residence times of cinnamic and ferulic acids were estimated to be 12 and 21 s at 573 K, based on time-resolved experiments. Cinnamic acid undergoes a CO2 elimination reaction at temperatures higher than 873 K without an intermediate liquid phase. At temperatures as low as 573 K, –OH and –OCH3 substituted cinnamic acids underwent decarboxylation despite bearing similar BDEs for Cβ–Cγ scission. At these temperatures, p-coumaric and ferulic acids were converted into 4-vinylphenol and 4-vinylguaiacol by 40 wt% and 30 wt%, respectively. On the other hand, sinapinic acid converted nearly by 80 wt% at temperatures below its boiling point of 676 K. In conjunction with novel quantum chemical calculations, it could be ruled out that decarboxylation was not occurring via concerted unimolecular reactions at low temperatures. Instead, water-catalyzed reactions of hydroxycinnamic acids seem to be the primary cause for the CO2 elimination in the intermediate liquid phase via a 6-centered transition state.